An object is a table (or map), associating properties or fields (of type string) to values (of type any). There is no need to declare any "class" or object template before creating objects:

let emptyObj = {};
let person = {
  last: "Bond",
  first: "James",
  age: 27,
};

Access to properties is (almost universally) accomplished with the . operator:

console.log(`My name is ${person.last}, ${person.first} ${person.last}`);

Mutating objects

An object is a dynamic structure, so properties can be added

person.weight = 80;

or deleted

delete person.age;

Adding and removing properties to objects confuses the JIT, so in general it is a good idea to keep the structure of objects fixed if one cares about performance.

Special names

Since internally the object is a table, properties do not need to be named like variable names. If they are they can be accessed normally, if not, they can be accessed with the brackets operator [].

let obj1 = {};
obj["full name"] = "Maximilian Schmitz";

This same notation can be used in initialization:

let obj2 = {
  ['full name']: "Maximilian Schmitz II";
}

Since initialization uses the : separator between properties and values, we can even drop the brackets in that case:

let obj3 = {
  'full name': "Maximilian Schmitz III";
}

This is the notation that gives rise to JSON.

Checking for existence

Since if you are given an object, in advance you don't know what properties it can have. You can ask if an object has a property with the in operator:

if ("first" in person) {
  console.log(`First name: ${person.first}`);
}

Iterating properties

If you need to go over all the properties of an object you can use a special loop: for in. The loop gives, at every iteration the name of one of the properties of the object as a string.

let obj = { x: 0.5, b: true, c: "hi" };
for (const prop in person) {
  console.log(`obj has property "${prop}" with value: ${obj[prop]});
}

Special property functions

Three functions let us query for an objects inner associations:

Abbreviations

When initializing an object:

let first = "James";
let last = "Bond";
let age = 27;
let person = {
  first: first,
  last: last,
  age: age,
};

it seems wasteful to repeat the names of variables if they come from independent variables with the same name. So Javascript lets you write this instead:

let first = "James";
let last = "Bond";
let age = 27;
// Take name and value from variable directly
let person = { first, last, age };